Chicken marsala is not a candidate for a slow cooker in my opinion. The breasts will be grossly overdone. I use chicken marsala as a recipe for a 10 minute meal actually.
Cooking with wine in a crockpot is fine.

This is a good time to bring up how alcohol does cook off. It does not all cook off quickly at all. And it should be remembered that if you are preparing something for a recovering alcoholic, the flavor is still very much there.

Now, it may be that the amount of alcohol in a dish is modest to start with, but the fact that some of the alcohol remains could be of significant concern to recovering alcoholics, parents, and others who have ethical or religious reasons for avoiding alcohol. Madeleine Kamman, James Peterson and some of our other cooking heroes may be on the mark when stating that the harshness of the alcohol burns off quickly. But the alcohol itself is clearly another matter.

My post above yours shows that it isn't just traces but actually quite a lot of alcohol remains.
I mentioned the flavor in particular because someone on another board who is in that situation mentioned that fact in particular.